Dynamics of real time DPOAE contralateral suppression in chinchillas and humans Dinámica de la supresión contralateral de las DPOAE en tiempo real en chinchillas y humanos

The effects of broadband elicitor duration on a psychoacoustic measure of cochlear gain reduction

Physiological and psychoacoustic studies of the medial olivocochlear reflex (MOCR) in humans have often relied on long duration elicitors (>100 ms). This is largely due to previous research using otoacoustic emissions (OAEs) that found multiple MOCR time constants, including time constants in the 100s of milliseconds, when elicited by broadband noise. However, the effect of the duration of a broadband noise elicitor on similar psychoacoustic tasks is currently unknown. The current study measured the effects of ipsilateral broadband noise elicitor duration on psychoacoustic gain reduction estimated from a forward-masking paradigm. Analysis showed that both masker type and elicitor duration were significant main effects, but no interaction was found. Gain reduction time constants were ∼46 ms for the masker present condition and ∼78 ms for the masker absent condition (ranging from ∼29 to 172 ms), both similar to the fast time constants reported in the OAE literature (70-100 ms). Maximum gain reduction was seen for elicitor durations of ∼200 ms. This is longer than the 50-ms duration which was found to produce maximum gain reduction with a tonal on-frequency elicitor. Future studies of gain reduction may use 150-200 ms broadband elicitors to maximally or near-maximally stimulate the MOCR.

A Time-Course-Based Estimation of the Human Medial Olivocochlear Reflex Function Using Clicks

The auditory efferent system, especially the medial olivocochlear reflex (MOCR), is implicated in both typical auditory processing and in auditory disorders in animal models. Despite the significant strides in both basic and translational research on the MOCR, its clinical applicability remains under-utilized in humans due to the lack of a recommended clinical method. Conventional tests employ broadband noise in one ear while monitoring change in otoacoustic emissions (OAEs) in the other ear to index efferent activity. These methods, (1) can only assay the contralateral MOCR pathway and (2) are unable to extract the kinetics of the reflexes. We have developed a method that re-purposes the same OAE-evoking click-train to also concurrently elicit bilateral MOCR activity. Data from click-train presentations at 80 dB peSPL at 62.5 Hz in 13 young normal-hearing adults demonstrate the feasibility of our method. Mean MOCR magnitude (1.7 dB) and activation time-constant (0.2 s) are consistent with prior MOCR reports. The data also suggest several advantages of this method including, (1) the ability to monitor MEMR, (2) obtain both magnitude and kinetics (time constants) of the MOCR, (3) visual and statistical confirmation of MOCR activation.

Behavioral Measures of Cochlear Gain Reduction Depend on Precursor Frequency, Bandwidth, and Level

Sensory systems adjust to the environment to maintain sensitivity to change. In the auditory system, the medial olivocochlear reflex (MOCR) is a known physiological mechanism capable of such adjustment. The MOCR provides efferent feedback between the brainstem and cochlea, reducing cochlear gain in response to sound. The perceptual effects of the MOCR are not well understood, such as how gain reduction depends on elicitor characteristics in human listeners. Physiological and behavioral data suggest that ipsilateral MOCR tuning is only slightly broader than it is for afferent fibers, and that the fibers feed back to the frequency region of the cochlea that stimulated them. However, some otoacoustic emission (OAE) data suggest that noise is a more effective elicitor than would be consistent with sharp tuning, and that a broad region of the cochlea may be involved in elicitation. If the elicitor is processed in a cochlear channel centered at the signal frequency, the growth of gain reduction with elicitor level would be expected to depend on the frequency content of the elicitor. In the current study, the effects of the frequency content and level of a preceding sound (called a precursor) on signal threshold was examined. The results show that signal threshold increased with increasing precursor level at a shallower slope for a tonal precursor at the signal frequency than for a tonal precursor nearly an octave below the signal frequency. A broadband noise was only slightly more effective than a tone at the signal frequency, with a relatively shallow slope similar to that of the tonal precursor at the signal frequency. Overall, these results suggest that the excitation at the signal cochlear place, regardless of elicitor frequency, determines the magnitude of ipsilateral cochlear gain reduction, and that it increases with elicitor level.

The effect of broadband elicitor laterality on psychoacoustic gain reduction across signal frequency

There are psychoacoustic methods thought to measure gain reduction, which may be from the medial olivocochlear reflex (MOCR), a bilateral feedback loop that adjusts cochlear gain. Although studies have used ipsilateral and contralateral elicitors and have examined strength at different signal frequencies, these factors have not been examined within a single study. Therefore, basic questions about gain reduction, such as the relative strength of ipsilateral vs contralateral elicitation and the relative strength across signal frequency, are not known. In the current study, gain reduction from ipsilateral, contralateral, and bilateral elicitors was measured at 1-, 2-, and 4-kHz signal frequencies using forward masking paradigms at a range of elicitor levels in a repeated measures design. Ipsilateral and bilateral strengths were similar and significantly larger than contralateral strength across signal frequencies. Growth of gain reduction with precursor level tended to differ with signal frequency, although not significantly. Data from previous studies are considered in light of the results of this study. Behavioral results are also considered relative to anatomical and physiological data on the MOCR. These results indicate that, in humans, cochlear gain reduction is broad across frequencies and is robust for ipsilateral and bilateral elicitation but small for contralateral elicitation.

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

The relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition at positive and negative signal-to-noise ratios

Active mechanisms that regulate cochlear gain are hypothesized to influence speech-in-noise perception. However, evidence of a relationship between the amount of cochlear gain reduction and speech-in-noise recognition is mixed. Findings may conflict across studies because different signal-to-noise ratios (SNRs) were used to evaluate speech-in-noise recognition. Also, there is evidence that ipsilateral elicitation of cochlear gain reduction may be stronger than contralateral elicitation, yet, most studies have investigated the contralateral descending pathway. The hypothesis that the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition depends on the SNR was tested. A forward masking technique was used to quantify the ipsilateral cochlear gain reduction in 24 young adult listeners with normal hearing. Speech-in-noise recognition was measured with the PRESTO-R sentence test using speech-shaped noise presented at -3, 0, and +3 dB SNR. Interestingly, greater cochlear gain reduction was associated with lower speech-in-noise recognition, and the strength of this correlation increased as the SNR became more adverse. These findings support the hypothesis that the SNR influences the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition. Future studies investigating the relationship between cochlear gain reduction and speech-in-noise recognition should consider the SNR and both descending pathways.

The effects of preceding sound and stimulus duration on measures of suppression in younger and older adults

Despite clinically normal audiometric thresholds, some older adults may experience difficulty in tasks such as understanding speech in a noisy environment. One potential reason may be reduced cochlear nonlinearity. A sensitive measure of cochlear nonlinearity is two-tone suppression, which is a reduction in the auditory system's response to one tone in the presence of a second tone. Previous research has been mixed on whether suppression decreases with age in humans. Studies of efferent cochlear gain reduction also suggest that stimulus duration should be considered in measuring suppression. In the present study, suppression was first measured psychoacoustically using stimuli that were too short to result in gain reduction. The potential effect of efferent cochlear gain reduction was then measured by using longer stimuli and presenting tonal or noise precursors before the shorter stimuli. Younger adults (ages 19-22 yr) and older adults (ages 57+ yr) with clinically normal hearing were tested. Suppression estimates decreased with longer stimuli or preceding sound which included the signal frequency, but did not decrease with preceding sound at the suppressor frequency. On average, the older group had lower suppression than the younger group, but this difference was not statistically significant.

Evidence for Gain Reduction by a Precursor in an On-Frequency Forward Masking Paradigm

A forward masking technique was used to measure cochlear gain reduction which might be consistent with the medial olivocochlear reflex (MOCR). A 4-kHz signal was set at 20 dB SL, and an on-frequency forward masker adjusted to just mask the signal. Adding a pink noise precursor before the signal and masker increased the level of the masker needed to mask the signal, in contrast to what would be expected from theories such as additivity of masking. The magnitude and pattern of this increase was similar to the increase in signal threshold seen with an off-frequency masker following a precursor.

Psychoacoustic measurements of ipsilateral cochlear gain reduction as a function of signal frequency

Forward masking experiments at 4 kHz have demonstrated that preceding sound can elicit changes in masking patterns consistent with a change in cochlear gain. However, the acoustic environment is filled with complex sounds, often dominated by lower frequencies, and ipsilateral cochlear gain reduction at frequencies below 4 kHz is largely unstudied in the forward masking literature. In this experiment, the magnitude of ipsilateral cochlear gain reduction was explored at 1, 2, and 4 kHz using forward masking techniques in an effort to evaluate a range of frequencies in listeners with normal hearing. Gain reduction estimates were not significantly different at 2 and 4 kHz using two forward masking measurements. Although the frequency was a significant factor in the analysis, post hoc testing supported the interpretation that gain reduction estimates measured without a masker were not significantly different at 1, 2, and 4 kHz. A second experiment provided evidence that forward masking in this paradigm at 1 kHz cannot be explained by excitation alone. This study provides evidence of ipsilateral cochlear gain reduction in humans at frequencies below the 4 kHz region.

Olivocochlear efferents: Their action, effects, measurement and uses, and the impact of the new conception of cochlear mechanical responses

The anatomy and physiology of olivocochlear (OC) efferents are reviewed. To help interpret these, recent advances in cochlear mechanics are also reviewed. Lateral OC (LOC) efferents innervate primary auditory-nerve (AN) fiber dendrites. The most important LOC function may be to reduce auditory neuropathy. Medial OC (MOC) efferents innervate the outer hair cells (OHCs) and act to turn down the gain of cochlear amplification. Cochlear amplification had been thought to act only through basilar membrane (BM) motion, but recent reports show that motion near the reticular lamina (RL) is amplified more than BM motion, and that RL-motion amplification extends to several octaves below the local characteristic frequency. Data on efferent effects on AN-fiber responses, otoacoustic emissions (OAEs) and human psychophysics are reviewed and reinterpreted in the light of the new cochlear-mechanical data. The possible origin of OAEs in RL motion is considered. MOC-effect measuring methods and MOC-induced changes in human responses are also reviewed, including that ipsilateral and contralateral sound can produce MOC effects with different patterns across frequency. MOC efferents help to reduce damage due to acoustic trauma. Many, but not all, reports show that subjects with stronger contralaterally-evoked MOC effects have better ability to detect signals (e.g. speech) in noise, and that MOC effects can be modulated by attention.

The effects of ipsilateral, contralateral, and bilateral broadband noise on the mid-level hump in intensity discrimination

Previous psychoacoustical and physiological studies indicate that the medial olivocochlear reflex (MOCR), a bilateral, sound-evoked reflex, may lead to improved sound intensity discrimination in background noise. The MOCR can decrease the range of basilar-membrane compression and can counteract effects of neural adaptation from background noise. However, the contribution of these processes to intensity discrimination is not well understood. This study examined the effect of ipsilateral, contralateral, and bilateral noise on the "mid-level hump." The mid-level hump refers to intensity discrimination Weber fractions (WFs) measured for short-duration, high-frequency tones which are poorer at mid levels than at lower or higher levels. The mid-level hump WFs may reflect a limitation due to basilar-membrane compression, and thus may be decreased by the MOCR. The noise was either short (50 ms) or long (150 ms), with the long noise intended to elicit the sluggish MOCR. For a tone in quiet, mid-level hump WFs improved with ipsilateral noise for most listeners, but not with contralateral noise. For a tone in ipsilateral noise, WFs improved with contralateral noise for most listeners, but only when both noises were long. These results are consistent with MOCR-induced WF improvements, possibly via decreases in effects of compression and neural adaptation.

Stronger efferent suppression of cochlear neural potentials by contralateral acoustic stimulation in awake than in anesthetized chinchilla

There are two types of sensory cells in the mammalian cochlea, inner hair cells, which make synaptic contact with auditory-nerve afferent fibers, and outer hair cells that are innervated by crossed and uncrossed medial olivocochlear (MOC) efferent fibers. Contralateral acoustic stimulation activates the uncrossed efferent MOC fibers reducing cochlear neural responses, thus modifying the input to the central auditory system. The chinchilla, among all studied mammals, displays the lowest percentage of uncrossed MOC fibers raising questions about the strength and frequency distribution of the contralateral-sound effect in this species. On the other hand, MOC effects on cochlear sensitivity have been mainly studied in anesthetized animals and since the MOC-neuron activity depends on the level of anesthesia, it is important to assess the influence of anesthesia in the strength of efferent effects. Seven adult chinchillas (Chinchilla laniger) were chronically implanted with round-window electrodes in both cochleae. We compared the effect of contralateral sound in awake and anesthetized condition. Compound action potentials (CAP) and cochlear microphonics (CM) were measured in the ipsilateral cochlea in response to tones in absence and presence of contralateral sound. Control measurements performed after middle-ear muscles section in one animal discarded any possible middle-ear reflex activation. Contralateral sound produced CAP amplitude reductions in all chinchillas, with suppression effects greater by about 1–3 dB in awake than in anesthetized animals. In contrast, CM amplitude increases of up to 1.9 dB were found in only three awake chinchillas. In both conditions the strongest efferent effects were produced by contralateral tones at frequencies equal or close to those of ipsilateral tones. Contralateral CAP suppressions for 1–6 kHz ipsilateral tones corresponded to a span of uncrossed MOC fiber innervation reaching at least the central third of the chinchilla cochlea.

Exploring the source of the mid-level hump for intensity discrimination in quiet and the effects of noise

Intensity discrimination Weber fractions (WFs) measured for short, high-frequency tones in quiet are larger at mid levels than at lower or higher levels. The source of this "mid-level hump" is a matter of debate. One theory is that the mid-level hump reflects basilar-membrane compression, and that WFs decrease at higher levels due to spread-of-excitation cues. To test this theory, Experiment 1 measured the mid-level hump and growth-of-masking functions to estimate the basilar membrane input/output (I/O) function in the same listeners. Results showed the initial rise in WFs could be accounted for by the change in I/O function slope, but there was additional unexplained variability in WFs. Previously, Plack [(1998). J. Acoust. Soc. Am. 103(5), 2530-2538] showed that long-duration notched noise (NN) presented with the tone reduced the mid-level hump even with a temporal gap in the NN. Plack concluded the results were consistent with central profile analysis. However, simultaneous, forward, and backward NN were not examined separately, which may independently test peripheral and central mechanisms of the NN. Experiment 2 measured WFs at the mid-level hump in the presence of NN and narrowband noise of different durations and temporal positions relative to the tone. Results varied across subjects, but were consistent with more peripheral mechanisms.

Influence of the stimulus presentation rate on medial olivocochlear system assays

Click evoked otoacoustic emissions (CEOAEs) are commonly used both in research and clinics to assay the medial olivocochlear system (MOC). Clicks presented at rates >50 Hz in the contralateral ear have previously been reported to evoke contralateral MOC activity. However, in typical MOC assays, clicks are presented in the ipsilateral ear in conjunction with MOC elicitor (noise) in the contralateral ear. The effect of click rates in such an arrangement is currently unknown. A forward masking paradigm was used to emulate typical MOC assays to elucidate the influence of ipsilateral click presentation rates on MOC inhibition of CEOAEs in 28 normal hearing adults. Influence of five click rates (20.83, 25, 31.25, 41.67, and 62.5 Hz) presented at 55 dB peSPL was tested. Results indicate that click rates as low as 31.25 Hz significantly enhance contralateral MOC inhibition, possibly through the activation of ipsilateral and binaural MOC neurons with potential contributions from the middle ear muscle reflex. Therefore, click rates ≤25 Hz are recommended for use in MOC assays, at least for 55 dB peSPL click level.

Time course of cochlear injury discharge (excitotoxicity) determined by ABR monitoring of contralateral cochlear events

The dynamics of cochlear excitotoxicity can be monitored from effects on the contralateral ear. After unilateral mechanical ablation of the cochlea (in a mouse model) we observed immediate elevations in auditory brainstem evoked response (ABR) thresholds in the contralateral ear. Threshold elevations peaked at 2-3 h post ablation, and returned to baseline levels after 5-6 h. These contralateral effects are initiated by cochlear afferent injury discharges most likely activating the olivocochlear efferent system. Six hours after cochlear injury, ABR thresholds were fully returned to pre-lesion baseline levels and remained normal for up to 10 days of monitoring. We have confirmed that our cochlear ablation procedure increases short-term activity levels in the auditory brainstem and midbrain using c-fos labelling. The study provides insight into the dynamics of glutamate excitotoxicity, a pathological process directly related to acute tinnitus after acoustic trauma, and more generally implicated in many types of brain injury and neuro-degenerative disease.

Age related changes to the dynamics of contralateral DPOAE suppression in human subjects

Background

The two ears are linked with a neural pathway such that stimulation of one ear has a modulating effect on the contralateral cochlea. This is mediated by cochlear afferent neurons connecting with olivo-cochlear efferents. The monitoring of this pathway is easily achieved by measuring contralateral suppression of otoacoustic emissions, and there is some clinical value in the ability to assess the integrity of this pathway. An important step in an evaluation of clinical utility is to assess any age-related changes. Accordingly, in the present study we measure the dynamics of contralateral DPOAE suppression in a population of normal hearing subjects of different ages.

Methods

Using a real-time DPOAE recording method we assessed contralateral DPOAE suppression in 95 ears from 51 subjects (age range 2–52 years). DPOAE (2f₁-f₂; f₂ = 4.4 kHz; f₂/f₁ = 1.22) input–output functions were measured. In response to contralateral broadband noise, dynamic aspects of DPOAE suppression were measured, specifically suppression onset latency and time constants.

Results

An age-related reduction in DPOAE amplitudes was observed. Both the detectability and the degree of contralateral DPOAE suppression were decreased in older age groups. We find an age-related increase in the latency of onset of DPOAE suppression to contralateral stimulation, but no significant change in suppression time-constants.

Conclusion

Olivo-cochlear function as revealed by contralateral suppression of DPOAEs shows some important age-related changes. In addition to reduced emissions (outer haircell suppression) we find an increased latency that may reflect deterioration in auditory brainstem function. Regarding clinical utility, it is possible that the changes observed may reflect an aspect of age-related hearing loss that has not been previously considered.

Contralateral ear occlusion for improving the reliability of otoacoustic emission screening tests

Newborn hearing screening is an established healthcare standard in many countries and testing is feasible using otoacoustic emission (OAE) recording. It is well documented that OAEs can be suppressed by acoustic stimulation of the ear contralateral to the test ear. In clinical otoacoustic emission testing carried out in a sound attenuating booth, ambient noise levels are low such that the efferent system is not activated. However in newborn hearing screening, OAEs are often recorded in hospital or clinic environments, where ambient noise levels can be 60–70 dB SPL. Thus, results in the test ear can be influenced by ambient noise stimulating the opposite ear. Surprisingly, in hearing screening protocols there are no recommendations for avoiding contralateral suppression, that is, protecting the opposite ear from noise by blocking the ear canal. In the present study we have compared transient evoked and distortion product OAEs measured with and without contralateral ear plugging, in environmental settings with ambient noise levels <25 dB SPL, 45 dB SPL, and 55 dB SPL. We found out that without contralateral ear occlusion, ambient noise levels above 55 dB SPL can significantly attenuate OAE signals. We strongly suggest contralateral ear occlusion in OAE based hearing screening in noisy environments.

Accounting for nonmonotonic precursor duration effects with gain reduction in the temporal window model

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

Separating the contributions of olivocochlear and middle ear muscle reflexes in modulation of distortion product otoacoustic emission levels

OBJECTIVES

Mediated by the medial olivocochlear system (MOCS), distortion product otoacoustic emission (DPOAE) levels are reduced by presentation of contralateral acoustic stimuli. Such acoustic signals can also evoke a middle ear muscle reflex (MEMR) that also attenuates recorded DPOAE levels. Our aim is to clearly differentiate these two inhibitory mechanisms and to analyze each separately, perhaps allowing the development of novel tests of hearing function.

METHODS

DPOAE were recorded in real time from chinchillas with normal auditory brainstem response thresholds and middle ear function. Amplitude reduction and its onset latency caused by contralateral presentation of intermittent narrow-band noise (NBN) were measured. Stapedius and tensor tympani muscle tendons were divided without disturbing the ossicular chain, and DPOAE testing was repeated.

RESULTS

Peak reduction of (2f1 - f2) DPOAE levels occurred when the center frequency of contralateral NBN approximated the primary tone f2, indicating an f2-frequency-specific response. For a 4.5-kHz centered NBN, DPOAE (f2 = 4.4 kHz) inhibition was 0.1 dB (p < 0.001). This response remained present after tendon division, consistent with an MOCS origin. Low-frequency NBN (center frequency: 0.5 kHz) reduced otoacoustic emission levels (0.1 dB, p < 0.001) across a wide range of DPOAE frequencies. This low-frequency response was abolished by division of the middle ear muscle tendons, clearly indicating MEMR involvement.

CONCLUSIONS

Following middle ear muscle tendon division, DPOAE inhibition by contralateral stimuli approximating the primary tone f2 persists, whereas responses evoked by lower contralateral frequencies are abolished. This distinguishes the different roles of the MOCS (f2 frequency specific) and MEMR (low frequency only) in contralateral modulation of DPOAE. This analysis helps clarify the pathways involved in an objective test that might have clinical benefit in the testing of neonates.

Postnatal maturation of contralateral DPOAE suppression in a precocious animal model (chinchilla) of the human neonate

CONCLUSION

In the neonatal chinchilla, the degree of contralateral distortion product otoacoustic emission (DPOAE) suppression and the latency and time constants of suppression are immature for 40-60 days. This suggests that olivocochlear efferent innervation of outer hair cells is not fully mature at birth in this animal model, and this may also be the case for human neonates.

OBJECTIVES

To track postnatal changes in the dynamics of the olivocochlear efferent system in an animal model with cochlear development at birth similar to that in humans.

METHODS

Real-time measurements of contralateral DPOAE suppression were made in 79 ears of anaesthetized chinchillas, ranging in age from 1 day to 70 days. An adult control group (13 ears) was also tested. DPOAE (2f1-f2; f2 = 4.4 kHz; f2/f1 = 1.22) input/output functions were measured. Dynamics of contralateral broadband noise suppression were measured, including latency and suppression time constants.

RESULTS

DPOAE amplitude input/output functions are immature until 20-30 days postnatally. The maturation period for contralateral suppression amplitude is about 30 days. Latency of onset suppression was 40 ms at birth reducing to adult values (23 ms) at 40 days. The DPOAE suppression time constant was about 350 ms at birth and mature (230 ms) at 60 days.

The time course of cochlear gain reduction measured using a more efficient psychophysical technique

In a previous study it was shown that an on-frequency precursor intended to activate the medial olivocochlear reflex (MOCR) at the signal frequency reduces the gain estimated from growth-of-masking (GOM) functions. This is called the temporal effect (TE). In Expt. 1 a shorter method of measuring this change in gain is established. GOM functions were measured with an on- and off-frequency precursor presented before the masker and signal, and used to estimate Input/Output functions. The change in gain estimated in this way was very similar to that estimated from comparing two points measured with a single fixed masker level on the lower legs of the GOM functions. In Expt. 2, the TE was measured as a function of precursor duration and signal delay. For short precursor durations and short delays the TE increased (buildup) or remained constant as delay increased, then decreased. The TE also increased with precursor duration for the shortest delay. The results were fitted with a model based on the time course of the MOCR. The model fitted the data well, and predicted the buildup. This buildup is not consistent with exponential decay predicted by neural adaptation or persistence of excitation.

On- and off-frequency compression estimated using a new version of the additivity of forward masking technique

On- and off-frequency compression at the 4000- and 8000-Hz cochlear places were estimated using a new version of the additivity of forward masking (AFM) technique, that measures the effects of combining two non-overlapping forward maskers. Instead of measuring signal thresholds to estimate compression of the signal as in the original AFM technique, the decrease in masker threshold in the combined-masker condition compared to the individual-masker conditions is used to estimate compression of the masker at the signal place. By varying masker frequency it is possible to estimate off-frequency compression. The maskers were 500-Hz-wide bands of noise, and the signal was a brief pure tone. Compression at different levels was estimated using different overall signal levels, or different masker-signal intervals. It was shown that the new AFM technique and the original AFM technique produce consistent results. Considerable compression was observed for maskers well below the signal frequency, suggesting that the assumption of off-frequency linearity used in other techniques may not be valid. Reducing the duration of the first masker from 200 to 20 ms reduced the compression exponent in some cases, suggesting a possible influence of olivocochlear efferent activity.

Distortion product otoacoustic emission contralateral suppression functions obtained with ramped stimuli

The purpose of this research was to investigate the changes that occur in human distortion product otoacoustic emission (DPOAE) level functions over continuous frequency bands in response to activation of the medial olivocochlear (MOC) efferent system by contralateral broadband noise. DPOAEs were obtained using continuous upward ramps of the lower frequency tone (f(1)) while the higher frequency tone (f(2)) was fixed. These ramps were designed to change the stimulus frequency ratio f(2)/f(1) over a fixed range for each fixed f(2) value of 2, 3, and 4 kHz. Contralateral noise was presented on alternating ramps and the DPOAEs with and without contralateral noise were averaged separately. Stimulus frequency ratios of 1.10 and 1.22, and noise levels of 60 and 50 dB sound pressure level (SPL) were employed. Changes in DPOAE level were generally suppression (a reduction in DPOAE magnitude), but enhancement was also observed. For most participants, changes were evident for much of the frequency ranges tested. Average absolute changes for 60 dB SPL noise were 0.95, 0.81, and 0.42 dB for the wider stimulus frequency ratios and f(2) of 2, 3, and 4 kHz, respectively. For the narrower ratio and 60 dB SPL noise, the changes were larger with average absolute changes of 1.33, 1.09, and 0.87 dB. For the narrower ratio and 50 dB SPL noise, the changes were 1.08, 0.78, and 0.55 dB with f(2) of 2, 3, and 4 kHz, respectively. DPOAE nulls were observed and a common response pattern was a shift of emission morphology to higher frequencies with contralateral acoustic stimulation. The method appears promising for relatively rapid evaluation of the MOC efferent system in humans and offers information complementary to measurement strategies that explore the effects of stimulus level.

Amplitude modulation of DPOAEs by acoustic stimulation of the contralateral ear

CONCLUSION

Otoacoustic emissions generated by outer hair cells (OHCs) are influenced by stimulation of the contralateral ear via a neural pathway involving the olivo-cochlear efferent system. This is often referred to as a contralateral 'suppression reflex', but we suggest that such a term is inappropriate since distortion product otoacoustic emissions (DPOAEs) can be both enhanced and suppressed, and there is continuous modulation with no threshold effects.

OBJECTIVE

To characterize the continuous amplitude modulation of DPOAEs by contralateral sound stimulation.

MATERIALS AND METHODS

In an animal model (chinchilla), DPOAEs were recorded in real time from one ear during presentation of acoustic stimuli to the opposite ear.

RESULTS

DPOAE amplitude is suppressed by an increase in contralateral stimulation, and enhanced by a decrease in same, i.e. the emissions are continuously modulated by activity in the opposite ear. The input-output function shows a linear relationship to this system over a 40-50 dB range of contralateral stimulus levels. After a neural delay time of approximately 25 ms, DPOAE amplitude closely follows contralateral amplitude signals up to modulation frequencies of approximately 20 Hz. Thus, stimuli to one ear continually modulate the OHC system (and therefore the biomechanical amplification) of the contralateral cochlea.

Selective attention to visual stimuli reduces cochlear sensitivity in chinchillas

It is generally accepted that during periods of attention to specific stimuli there are changes in the neural activity of central auditory structures; however, it is controversial whether attention can modulate auditory responses at the cochlear level. Several studies performed in animals as well as in humans have attempted to find a modulation of cochlear responses during visual attention with contradictory results. Here, we have appraised cochlear sensitivity in behaving chinchillas by measuring, with a chronically implanted round-window electrode, sound-evoked auditory-nerve compound action potentials and cochlear microphonics, a measure of outer hair cell function, during selective attention to visual stimuli. Chinchillas were trained in a visual discrimination or in an auditory frequency discrimination two-choice task. We found a significant decrease of cochlear sensitivity during the period of attention to visual stimuli in the animals performing the visual discrimination task, but not in those performing the auditory task, demonstrating that this physiological effect is related to selective attention to visual stimuli rather than to an increment in arousal level. Furthermore, the magnitude of the cochlear-sensitivity reductions increased in sessions performed with shorter target-light durations (4-0.5 s), suggesting that this effect is stronger for higher attentional demands of the task. These results demonstrate that afferent auditory activity is modulated by selective attention as early as at sensory transduction, possibly through activation of olivocochlear efferent fibers.

Short-Term Pathophysiologic Changes and Histopathologic Findings of the Auditory Pathway after Closed Head Injury, Using a Rabbit Model

Hearing impairment is a well-known consequence of closed head injury (CHI). The aim of this study was to elucidate the pathogenesis of CHI-induced hearing loss, using a rabbit model. Twelve New Zealand white rabbits were divided into two groups of 6. In the first group, CHI was induced mechanically, whereas the rabbits of the second group served as controls. Baseline distortion product otoacoustic emissions (DPOAEs), contralateral suppression (CS) of the DPOAEs and auditory brainstem response (ABR) were obtained. The same measurements were performed in the first group after CHI. Three hours later, the animals were sacrificed and their brain was excised and subjected to histopathologic examination. Mean I-III ABR latencies were increased and DPOAE amplitudes and CS values were reduced in the trauma group after CHI, at a statistically significant level. Histopathologic examination of the temporal lobe and brainstem showed multiple hemorrhagic and necrotic areas, with edema in the surrounding region. The vestibulocochlear nerve was severely damaged at its emerging site at the brainstem. In conclusion, both peripheral and central involvement of the auditory pathway was found after CHI. Otoacoustic emissions in conjunction with ABR may provide significant information on both peripheral and central auditory function.